The photoacoustic technique is based upon the work of A. G. Bell c. 1880 which resulted in the discovery that substances which are periodically illuminated by light can emit an acoustic or sound signal.
More modern work with this principle has resulted in the development of photoacoustic spectroscopy (see &lt;&gt; in Optoacoustic Spectroscopy and Detection, Yo Han Pao (Ed.) &lt; A. Rosencwaig&gt;, Academic Press, NY 1977) and the development since then of commercial apparatus for carrying out optoacoustic or photoacoustic spectroscopy.
In this technique, the sample is placed in a hermetically sealed cell connected to a microphone by an acoustic coupling medium such as a carrier gas filling the space between the sample and the microphone and forming a sound-transmitting medium.
The sample is irradiated with modulated light with modulation frequencies ranging from several Hz to several kHz.
The light absorbed by the sample produces heat which spreads in the sample as heat waves reaching the sample surface where these waves are transformed to compression and rarefaction waves of the adjacent thin layer of carrier gas.
Because of the periodic nature of the light beam directed at the sample and generated by a chopper, the heat waves are generated at a corresponding frequency and the alternating periodic heating of the gas layer at the sample surface results in pressure fluctuations which constitute alternating cycles of condensation and rarefaction, i.e. sound signals which are picked up by the microphone.
The amplified microphone signal is fed to a phase-sensitive lock-in amplifier and the reference frequency of the lock-in amplifier is made identical to the modulation frequency of the light source. The signal output can be plotted as a photoacoustic signal spectrum in which, for instance, the photoacoustic signal amplitude is plotted as the ordinate against the light frequency or wavelength along the abscissa.
The resulting photoacoustic spectrum is an indication of the absorption and thermodynamic characteristics of the sample material.
Photoacoustic spectroscopy can be used in chemistry, physics, biology and medicine and indeed wherever determinations of properties of materials must be made.
Generally the measurements are carried out at room temperature although cooled samples and especially measurements at cryogenic temperatures have been employed.
In the latter case, the sample-receiving vessel has been immersed in a bath of liquid nitrogen.
The boiling of the liquid cryogen, e.g. the liquid nitrogen, has been found to generate measurement distortions (perturbations and noise) which affect the results obtained and the accuracy of the measurements.